1997 Dakota 125000 miles , noticed a high pitch sound at 40 mph or higher, starts when driving .

 

What kind of sound? What area of the vehicle is it coming from? The more information you provide, the easier it is for us to assist you instead of having to ask you a bunch of questions. .

 

Sounds like its coming from Trans to rear axle , the sound will aggravate your hearing. This is my Dads truck , had the front wheel bearings replaced about 6 months ago the shop thought that was making the noise. I drove the truck yesterday and noticed the sound .V-6 auto RWD . The sound starts at 40 MPH and faster.
Thank you
LMG

 

Run it on jack-stands and see if u can pinpoint from underneath. Have you checked rear diff fluid? Is it 4wd or rwd?